The present invention relates to a processing method for radiation image signals, and particularly to a technology for obtaining an excellent object image, by which a dynamic range, frequency characteristics, and gradations are appropriately processed in order to improve diagnostic performance.
A radiation image such as an X-ray image is widely used for medical purposes. Conventionally, in order to obtain the X-ray image, a radiograph is widely used in which a fluorescent screen (a fluorescent substance layer) is irradiated with the X-ray passed through an object, a visible light corresponding to a penetrating radiation dose is generated thereby, a film, on which silver salt is used, is irradiated with the visible light, and the film is developed in the same way as an ordinary photograph.
However, recently, a method has been invented by which image information is directly read out from the fluorescent substance layer without using a film on which silver salt is coated.
As an above-mentioned method, the following method is used: a radioactive ray, passed through the object, is absorbed in a stimulable fluorescent substance; after that, the stimulable fluorescent substance is excited by, for example, light or thermal energy; thereby, radiation energy (radiation image information) accumulated in the stimulable fluorescent substance by the adsorption is stimulatingly emitted; and the stimulation emission light is photoelectrically converted and an image signal is obtained.
Specifically, a radiation image conversion method, in which a stimulable fluorescent substance is used, and a visible light or an infrared ray is used as a stimulable excitation light, is disclosed in, for example, U.S. Pat. No. 3,859,527 and Japanese Patent Publication Open to Public Inspection No. 12144/1980. In these disclosures, the following operations are carried out: a stimulable fluorescent panel, in which the stimulable fluorescent substance layer is formed on a support, is used; the stimulable fluorescent substance layer is irradiated with the radioactive rays which have passed through an object; radiation energy corresponding to radioactive ray transmittance of each portion of the object is accumulated to form a latent image; then, the stimulable fluorescent substance layer is scanned by the stimulable excitation light; thereby, the radiation energy, which is accumulated in the fluorescent panel, is radiated and converted into a light signal; and the light signal is photoelectrically converted so that a radiation image signal can be obtained.
As described above, in the system by which the radiation image signal is obtained, and a visible image is obtained according to the image signal, the radiation image signal is processed so that a visible image, which is superior in diagnostic properties, is obtained.
For example, the following processes are carried out: a dynamic range of an original image signal is compressed (refer to Japanese Patent Publication Open to Public Inspection Nos. 189854/1988, 222577/1991); and frequencies are emphasized (refer to Japanese Patent Publication Open to Public Inspection No. 163472/1980).
Dynamic range compression processing is effective for spreading a signal range in which a contrast is acceptable for diagnosis. However, when the compressed region is observed, the contrast of the region is lowered, and the diagnostic properly is lowered. That is, when it is intended to emphasize the contrast in order to improve the diagnostic property for region of interest, dynamic range extension processing muse be carried out instead of compression processing. However, when the extension processing is carried out, the dynamic range is spread, therefore, low density regions become while, the high density region become black. Accordingly, sometimes, information disappears and a range, in which a diagnosis can be carried out, is narrowed. Further, sometimes, the difference between the density of the original image signal and that of the processed image signal becomes large, and accordingly, an accurate reproduction can not be made.
Further, in the image in which a dynamic range of some frequency components of the signals is compressed other (non-compressed) frequency components of the signals, is sometimes seen in a visual sense as if the contrast were lowered. Furthermore, in the image, some regions off which is compressed, the contrast of the other (non-compressed) regions is sometimes seen in a visual sense as if it were lowered, so that the diagnostic property is adversely affected.